1. Field of the Invention
The invention relates to a method of producing a heating element which has a predetermined resistance value and a high heat-up rate. Such heating elements are desirable, for example, for igniting propellants for airbag systems. At present, such heating elements are made from resistor wires, with the diameter of the wires selected to be very thin (approximately 10 .mu.m) in order to attain a high heat-up rate. The resistance value of a specified resistor wire having a predetermined wire length can only be varied by changing the cross-section of the wire. Since a wide spectrum of resistive values should be included, technical limits relating to the heat-up rate, ease of manipulation and installation of the wire will be reached quickly.
2. Description of the Related Art
U.S. Pat. No. 3,998,980 describes a thick-film resistor for thermal printer applications formed as a pixel element with a predetermined resistance value. The thick-film resistor consists of several printed layers applied on a ceramic substrate which is coated with a crystallizing glass forming a heat barrier. The resistor has a thickness in a range between 12.5 .mu.m and 254 .mu.m. The resistive material is a paste made of bismuth ruthenate. The smooth surface characteristics required of resistors used for printer elements can be obtained by lapping the resistor, with lapping being performed each time a printed layer has been applied or alternatively also as the last step in the process. Lapping is also used to adjust the resistance value and the temperature coefficient of the resistor. A subsequent annealing process is performed to prevent micro-cracks from developing in the resistor layer which could result in an increase in the resistance with aging. Disadvantageously, however, this type of resistor is formed as a thick-film resistor rather than as a thin-film resistor. Due to the large heat capacity of the resistor, the heat-up rate cannot be decreased below a certain value.
EP 0 471 138 A2 describes a method of producing an electric precision resistor with a predetermined temperature coefficient, wherein a platinum thin-film is applied on an aluminum oxide ceramic substrate. A layer consisting of a mixture which includes platinum resinate and rhodium resinate, is subsequently applied on the platinum thin-film, with the rhodium content determining the desired temperature coefficient. The coated substrate is subjected to a thermal treatment in the range of 1000.degree. C. to 1400.degree. C. until the rhodium is uniformly distributed in the formed resistor layer. The layer has a rhodium content in a range between 0.1% and 12% with respect to the combined content of platinum and rhodium. The temperature coefficient of precision resistors based on platinum alloys can be precisely adjusted in a range between 1600 and 3850 ppm/K by varying the rhodium content of the resistor layer. This method, however, is not intended to allow a precise adjustment the specific sheet resistance of the resistor layer.
WO 96/01983 A1 describes a method of producing a sensor for measuring temperature and/or flow, wherein the sensor is formed of a patterned resistor layer on a substrate. The resistor layer is a platinum rhodium film made of an annealed mixture of platinum resinate and rhodium resinate. For example, a platinum-rhodium resistor layer with a temperature coefficient of 3500 ppm/.degree. C. can be realized by using a mixture of 99% platinum resinate paste and 1% rhodium paste. This method is also not intended to exactly adjust the specific sheet resistance of the resistor layer.
EP 0 576 017 A2 describes a method of producing an inkjet print head, wherein a thin-film layer forms a heating element which is heated to a temperature of 300.degree. C. within several microseconds, and subsequently cools down again to room temperature. The contact areas of the thin-film heating elements are made of a Au resinate or a Pt resinate paste. These contact areas cannot be soldered. The thin film is made of a resinate paste containing, for example, metal alloys such as WNi, ZrCr, Talr, TaFe or ZrNi. The main emphasis is here on the compatibility with ink; the reference does not address adjusting the specific sheet resistance.
DE-OS-2 020 016 discloses igniting means formed as a metal layer and disposed on an isolating element made of glass or ceramic. Two contact areas are applied to the isolating element by screen printing, using palladium-palladium silver from a palladium-gold, palladium-silver, nickel or silver-aluminum thick-film conductive paste, which is subjected to a sintering process between 1000.degree. C. and 1100.degree. C. A tantalum or tantalum nitride layer is subsequently evaporated and patterned by a photo lithographic process to form an ignition bridge which overlaps the marginal zones of the two contact areas. The length and width of the ignition bridge preferably ranges from 50 to 100 .mu.m, with the thickness ranging from 0.2 .mu.m to 1.5 .mu.m. Disadvantageously, this process is rather complex because two different techniques, namely a thick-film technique (screen printing) and a thin-film technique (evaporation), are used. The photolithographic process for structuring the ignition bridge introduces additional problems in that the applied thick-film contact areas adversely affect the planarity of the surface. This unevenness can cause an insufficient exposure during contact printing processes which has the disadvantage of poorly reproducing the structure of the ignition bridge element.
The length of the heating element used to ignite propellants for airbag systems is specified by the dimensions available for installation in a suitable housing. When the layer thickness is specified, the resistance value of the resistor path can be increased only by reducing the width of the resistor. The width of the resistor cannot be decreased below a certain value because the resistor must have a minimum area to transfer the heat for reliably igniting the propellant.